The phenomenon of field emission was discovered in the 1950's, and extensive research by many individuals, such as Charles A. Spindt of SRI International, has improved the technology to the extent that its use in the manufacture of inexpensive, low-power, high-resolution, high-contrast, full-color flat displays is possible. Advances in field emission display technology are disclosed in U.S. Pat. No. 3,755,704, "Field Emission Cathode Structures and Devices Utilizing Such Structures," issued 28 Aug. 1973, to C. A. Spindt et al.; U.S. Pat. No. 4,940,916, "Electron Source with Micropoint Emissive Cathodes and Display Means by Cathodoluminescence Excited by Field Emission Using Said Source," issued 10 Jul. 1990 to Michel Borel et al.; U.S. Pat. No. 5,194,780, "Electron Source with Microtip Emissive Cathodes," issued 16 Mar. 1993 to Robert Meyer; and U.S. Pat. No. 5,225,820, "Microtip Trichromatic Fluorescent Screen," issued 6 Jul. 1993, to Jean-Frederic Clerc. These patents are incorporated by reference into the present application.
A FED flat panel display arrangement is disclosed in U.S. Pat. No. 4,857,799, "Matrix-Addressed Flat Panel Display," issued Aug. 15, 1989, to Charles A. Spindt et al., incorporated herein by reference. This arrangement includes a matrix array of individually addressable light generating means of the cathodoluminescent type having electron emitting cathodes combined with an anode which is a luminescing means which reacts to electron bombardment by emitting visible light. Each cathode is itself an array of thin film field emission cathodes on a backing plate, and the luminescing means is provided as a phosphor coating on a transparent face plate which is closely spaced to the cathodes.
The emitter backing plate disclosed in the Spindt et al. ('799) patent includes a large number of vertical conductive cathode electrodes which are mutually parallel and extend across the backing plate and are individually addressable. Each backing plate includes a multiplicity of spaced-apart electron emitting tips which project upwardly from the vertical cathode electrodes on the backing plate and therefore extend perpendicularly away from the backing plate. An electrically conductive gate electrode arrangement is positioned adjacent to the tips to generate and control the electron emission. The gate electrode arrangement comprises a large number of individually addressable, horizontal electrode stripes which are mutually parallel and extend along the backing plate orthogonal to the cathode electrodes, and which include apertures through which emitted electrons may pass. Each gate electrode is common to a full row of pixels extending across the front face of the backing plate and is electrically isolated from the arrangement of cathode electrodes. The emitter back plate and the anode face plate are parallel and spaced apart.
The anode is a thin film of an electrically conductive transparent material, such as indium tin oxide (ITO), which covers the interior surface of the anode face plate. Deposited onto this metal layer is a luminescent material, such as phosphor, that emits light when bombarded by electrons.
The array of emitting tips on the backing plate is activated by addressing the orthogonally related cathode gate electrodes in a generally conventional matrix-addressing scheme. The appropriate cathode electrodes of the display along a selected stripe, such as along one column, are energized while the remaining cathode electrodes are not energized. Gate electrodes of a selected stripe orthogonal to the selected cathode electrode are also energized while the remaining gate electrodes are not energized, with the result that the emitting tips of a pixel at the intersection of the selected cathode and gate electrodes will be simultaneously energized, emitting electrons so as to provide the desired pixel display.
The Spindt et al. patent teaches that it is preferable that an entire row of pixels be simultaneously energized, rather than energization of individual pixels. According to this scheme, sequential lines are energized to provide a display frame, as opposed to sequential energization of individual pixels in a raster scan manner.
The Clerc ('820) patent discloses a trichromatic field emission flat panel display having a first substrate comprising the cathode and gate electrodes, and having a second substrate facing the first, including regularly spaced, parallel conductive stripes comprising the anode electrode. These stripes are alternately covered by a first material luminescing in the red, a second material luminescing in the green, and a third material luminescing in the blue, the conductive stripes covered by the same luminescent material being electrically interconnected.
Today, a conventional FED is manufactured by combining the teachings of many practitioners, including the teachings of the Spindt et al. ('799) and Clerc ('820) patents. Referring initially to FIG. 1, there is shown, in cross-sectional view, a portion of an illustrative prior field emission device. This device comprises an anode plate 1 having an electroluminescent phosphor coating 3 facing an emitter plate 2, the phosphor coating 3 being observed from the side opposite to its excitation.
More specifically, the field emission device of FIG. 1 comprises a cathodoluminescent anode plate 1 and an electron emitter (or cathode) plate 2. A cathode portion of emitter plate 2 includes conductors 9 formed on an insulating substrate 10, an electrically resistive layer 8 which is formed on substrate 10 and overlaying the conductors 9, and a multiplicity of electrically conductive microtips 5 formed on the resistive layer 8. In this example, the conductors 9 comprise a mesh structure, and microtip emitters 5 are configured as a matrix within the mesh spacings. Microtips 5 take the shape of cones which are formed within apertures through conductive layer 6 and insulating layer 7.
A gate electrode comprises the layer of the electrically conductive material 6 which is deposited on the insulating layer 7. The thicknesses of gate electrode layer 6 and insulating layer 7 are chosen in such a way that the apex of each microtip 5 is substantially level with the electrically conductive gate electrode layer 6. Conductive layer 6 may be in the form of a continuous layer across the surface of substrate 10; alternatively, it may comprise conductive bands across the surface of substrate 10.
Anode plate 1 comprises a transparent, electrically conductive film 12 deposited on a transparent planar support 13, such as glass, which is positioned facing gate electrode 6 and parallel thereto, the conductive film 12 being deposited on the surface of the glass support 13 directly facing gate electrode 6. Conductive film 12 may be in the form of a continuous layer across the surface of the glass support 13; alternatively, it may be in the form of electrically isolated stripes comprising three series of parallel conductive bands across the surface of the glass support 13, as shown in FIG. 1 and as taught in U.S. Pat. No. 5,225,820, to Clerc. By way of example, a suitable material for use as conductive film 12 may be indium-tin-oxide (ITO), which is optically transparent and electrically conductive. Anode plate 1 also comprises a cathodoluminescent phosphor coating 3, deposited over conductive film 12 so as to be directly facing and immediately adjacent gate electrode 6. In the Clerc patent, the conductive bands of each series are covered with a particulate phosphor coating which luminesces in one of the three primary colors, red, blue and green 3.sub.R, 3.sub.B, 3.sub.G.
Selected groupings of microtip emitters 5 of the above-described structure are energized by applying a negative potential to cathode electrode 9 relative to the gate electrode 6, via voltage supply 19, thereby inducing an electric field which draws electrons from the apexes of microtips 5. The potential between cathode electrode 9 and gate electrode 6 is approximately 70-100 volts. The freed electrons are accelerated toward the anode plate 1 which is positively biased by the application of a substantially larger positive voltage from voltage supply 11 coupled between the cathode electrode 9 and conductive film 12 functioning as the anode electrode. The potential between cathode electrode 9 and anode electrode 12 is approximately 300-800 volts. Energy from the electrons attracted to the anode conductive film 12 is transferred to particles of the phosphor coating 3, resulting in luminescence. The electron charge is transferred from phosphor coating 3 to conductive film 12, completing the electrical circuit to voltage supply 11. The image created by the phosphor stripes 3 is observed from the anode side which is opposite to the phosphor excitation, as indicated in FIG. 1.
It is to be noted and understood that true scaling information is not intended to be conveyed by the relative sizes and positioning of the elements of anode plate 1 and the elements of emitter plate 2 as depicted in FIG. 1. For example, in a typical FED shown in FIG. 1 there are approximately one hundred arrays 4, of microtips per display pixel, and there are three color stripes 3.sub.R, 3.sub.B, 3.sub.G per display pixel.
The process of producing each frame of a display using a typical trichromatic field emission display includes (1) applying an accelerating potential to the red anode stripes while sequentially addressing the gate electrodes (row lines) with the corresponding red video data for that frame applied to the cathode electrodes (column lines); (2) switching the accelerating potential to the green anode stripes while sequentially addressing the rows lines for a second time with the corresponding green video data for that frame applied to the column lines; and (3) switching the accelerating potential to the blue anode stripes while sequentially addressing the row lines for a third time with the corresponding blue video data for that frame applied to the column lines. This process is repeated for each display frame.
All red stripes 3.sub.R of the anode plate 1 are electrically coupled together. All green stripes 3.sub.G and all blue stripes 3.sub.B are also electrically coupled to each other. The prior art structure used to facilitate the electrical interconnection of the color anode stripes 3.sub.R, 3.sub.G, and 3.sub.B, is shown in FIGS. 2 and 3. FIG. 2 shows the manner in which the conductive film 34 of the anode stripes are interconnected in the prior art. The conductive films 34 are substantially similar to the conductive film 12 of FIG. 1. Conductive film 34.sub.R is covered with a phosphor coating luminescing in red, conductive film 34.sub.B is covered with a phosphor coating luminescing in blue, and conductive film 34.sub.G is covered with a phosphor coating luminescing in green.
The conductive films 34.sub.R are electrically interconnected by a first conductive band 36. The conductive films 34.sub.G are electrically interconnected by a second conductive band 38. The conductive films 34.sub.B are electrically interconnected by a anisotropic conductive ribbon 40 described more fully below. The first and second conductive bands 36, 38 are formed on the anode plate 1 at the same time the conductive films 34 are formed. The conductive bands 36, 38 and the conductive films 34 are also coplanar and both are comprised of the same conductive material, illustratively indium-tin-oxide (ITO).
The conductive films 34.sub.R which are connected to band 36 are interdigitated with the conductive films 34.sub.G which are connected to band 38 and the conductive films 34.sub.B which are connected to band 40. The anisotropic conductive ribbon 40 is deposited perpendicular to the conductive films 34.
FIG. 3 shows a section of the anode plate 1 along the anisotropic conductive ribbon 40, as indicated in FIG. 2. The anisotropic ribbon 40 is essentially formed by a conductive strip 40" and a film 40'. The film 40' comprises carbide balls 42 distributed in an insulating binder forming the film 40', so as not to conduct electricity. As can be seen from FIG. 3, the conductive strip 40" crushes the film 40' at the conductive films 34.sub.B. The density of the balls 42 is such that at the crushed points the balls 42 are in contact and the ribbon 40 becomes conductive at these points. Thus, the conductive films 34.sub.B are electrically connected to the conductive ribbon 40", but the non-crushed locations of film 40' are insulating.
There are numerous disadvantages to the prior art structure used to interconnect the red, green, and blue anode stripes. First, the use of the externally attached anisotropic ribbon 40 to connect the conductive films 34.sub.B creates a significant FED system reliability problem. If the ribbon 40 isn't assembled to anode plate 1 properly then the conductive films 34 of two or three colors will be shorted together. Furthermore, the ribbon 40 can become disconnected from the conductive films 34.sub.B, causing lines to appear in the display image at the places where the conductive films 34.sub.B are not electrically interconnected to the ribbon 40.
Two additional shortcomings of field emission displays of the prior art are the low contrast ratio of the display and the low emission intensity of the low voltage phosphors typically used as the luminescent material on the display screen. Referring briefly to FIG. 1, the low contrast ratio is due in part to phosphors which collect on the substrate 13 alongside the edges of the ITO anode stripes 12. This build-up of phosphors causes the delineation between the anode stripes to blur, thereby decreasing the clarity of the display image.
The low emission intensity of the phosphor has several origins, one of which is the low acceleration voltage used to excite the free electrons toward the anode. Currently, this acceleration voltage is limited by the potential which can be placed on the transparent stripe anode conductors underlaying the phosphor stripes, typically at about 300 volts. It is known that significantly improved performance would be provided by increasing the anode potential to about 1000 volts. However, as the acceleration voltage is increased, the leakage current between the conductive anode stripes also increases, eventually leading to breakdown when the leakage current becomes excessive. The sources of this leakage current between adjacent anode stripes include residual interstitial traces of anode conductor material which are not completely removed during the fabrication of the stripes, field emission from the stripe edges which are sharpened during fabrication, and the smooth glass surface of the substrate itself.
In view of the above, it is clear that there exists a need for a method of fabricating a more reliable bus structure. It is also clear that there exists a need for a method for fabricating an improved anode plate of a field emission flat panel display device which facilitates improved electrical and optical anode stripe delineation.